The short necks won't be dangerous to use, but your accuracy may suffer. The necks won't have as much purchase as the specified length will so bullets may not seat as straight as they should or hold as firmly. How come you trimmed them so short? I had the stop collar on my Forster trimmer loosen up one time and I overtrimmed 2 or 3 before I noticed. I think I just tossed them so I wouldn't get them mixed in with my other cases.